Energy curable coating and ink compositions are typically composed of mixtures of acrylated derivatives such as oligomeric acrylates and monomeric acrylates. In most instances, the monomeric materials are employed in the composition to control the viscosity of the coating or ink formulation depending on the particular method of application. However, these monomers often do not react completely during polymerization upon energy curing. Unreacted monomers remain as residual components in the dried printing ink or coated film and are subject to migration by absorption as well as surface contact. This migration of residual components can lead to a host of problems such as “odor” and “off-taste” in sensitive packaging applications such as food packaging. Alternatively, solvents are used to reduce or manipulate the formulation viscosity for appropriate applications. However, the use of solvents is often associated with unacceptable emissions, toxicity, and/or odor levels for more sensitive product applications.
The undesirable characteristics of residual solvents and monomers in specialized coatings and inks has spurred advancement of water based, energy curable compositions, aqueous based curing, and the development of energy curable processes in the presence of water. While typically a poor solvent for organic compounds and having too high surface tension to wet many polymer substrates, water in this instance may nevertheless be the ideal solvent for coating and ink delivery, able to lower viscosity and volatilize without adding to emissions, toxicity, or odor. The challenge is to formulate water compatibility over a wide range of compositions without producing water sensitivity and low rub resistance after curing.
An example of an energy curable composition can be found in EP 287,019. This reference, describes a composition wherein the oligomer is a carboxylic acid containing reaction product of a styrene maleic anhydride copolymer and a hydroxy(meth)acrylate. The composition further contains an ethylenically-unsaturated reactive diluent, a photoinitiator, and optionally a thiol. Exposure of the composition to an actinic source, e.g. a UV light source, results in an aqueous-developable material useful in making printing plates and photoresists. Such a composition would be less useful as a protective coating or a binder in ink specifically due to the designed sensitivity to aqueous development which would lead to low rub resistance when contacted by water.
Another example of an aqueous developable, energy curable composition can be found in EP 287,020. This reference describes an oligomeric material as the reaction product of a mono(meth)acrylate derivative of a caprolactone diol and styrene-maleic anhydride copolymer. The composition further optionally contains a reactive diluent and a photoinitiator. Exposure of the composition to a source of actinic radiation results in a solid cured product useful for making printing plates and photoresists wherein the exposed compositions are developed using an alkaline aqueous developer. Again, such a composition would be less useful as a protective coating or ink binder due to its water sensitivity. In neither of the above cases is delivery of the composition by aqueous solution actually described.
U.S. Pat. No. 5,665,840 discloses a water soluble, crosslinkable prepolymer having in its copolymer chain, as monomeric structural units, a vinyl lactam; a vinyl alcohol; optionally a lower alkane number carboxylic acid vinyl ester; a vinyl crosslinking agent; and optionally a vinylic photoinitiator. This reference also discloses a process for making prepolymers, as well as crosslinked, water insoluble, polymeric networks particularly useful for making hydrogels and water absorbing, molded articles such as contact lenses. Because these cross-linked, water insoluble, polymeric networks swell with water, they would be unsuitable as cured protective coatings and ink vehicles where they would exhibit low resistance to mechanical abrasion when in the presence of moisture.
U.S. Pat. No. 4,745,138 discloses a class of low molecular weight, partial esters of anhydride containing copolymers capable of providing non-aqueous, energy curable, liquid compositions for production of radiation-hardenable coatings without the need to employ an inert organic solvent. These compositions employ monomers containing terminally ethylenically unsaturated groups and maleic anhydride copolymers characterized by having free anhydride functionalities and are said to be particularly suitable for improving adhesion and the dispersive capabilities of binder resins. The partial esters are produced by esterifying a fraction of the anhydride groups by ring-opening with a hydroxyalkyl acrylic compound or an admixture thereof with a monohydric alkyl alcohol. By virtue of the introduction of hydrophobic substituents (particularly the esters of monohydric alkyl alcohols) and the absence of carboxylic acid groups, these compositions cure to films which are more water- and solvent-resistant than those made in accordance with the previous references. However, not discussed in this patent are aqueous solutions of these polymers as provided by hydrolysis of the residual anhydride in dilute caustic, the use of these solutions to stabilize solutions or colloidal dispersions of other, less polar materials, or coating or ink compositions prepared with these solutions.
A parallel approach uses solutions of acrylated, hydrophilic oligomers alone or together with the fore-mentioned polymers. Acrylated oligomers (and solutions of polymer resins made with oligomers) have a viscosity that is typically too high to be used directly for making coatings and printing inks. The use of water as a diluent to lower the viscosity of energy curable, acrylated, oligomeric mixtures has been described in U.S. Pat. No. 6,011,078 wherein the mixtures are used for wood and floor coating applications. The formulations taught in this patent are dispersions or emulsions and require prior evaporation of water followed by exposure to a temperature above the minimum film formation temperature (MFFT) before exposure to the actinic source. Without film formation prior to cure, the resultant energy cured, crosslinked polymer has very weak coherence, lacks adherence to a substrate, and does not provide the rub resistance required. Further, the additional drying step(s) slow the press speed and increase the potential for causing surface defects (e.g., lower gloss).
Acrylic functional polyesters containing salt structures are described by M. Philips, J. M. Loutz, S. Peeters, L. Lindekens, Polymers Paint Colour J., 183, #4322, p. 38 (1993). These are combined with hydrophilic monomers (e.g., polyethyleneglycol diacrylates) and water to make radiation curable, protective coatings. The combinations are described as homogeneous solutions that can be coated and radiation cured by UV with water-soluble photoinitiators to give rub- and wash-resistant top-coats. Also, see J. M. Loutz, S. Peeters, L. Lindekens, J. Coated Fabrics, 22, p. 298 (1993). In reality, all these formulations are very limited in the amount of water that can be incorporated and are comprised of high resin mass fraction (greater than 65 wt. % of vehicle) with consequently high viscosity. Typically, greater than 30 wt. % water on a total liquids (vehicle) basis causes degraded performance in the examples provided. Due to this fact, less than 10 wt. % water is recommended; and even at this water content, “a thermal flash-off step is recommended in order to avoid the formation of microporosity in the film.”
Recent discoveries demonstrated the use of ternary system that make it possible to print and cure the inks in one step without prior water evaporation. These inks contain ethylenically unsaturated oligomer and resins and have the properties of high gloss, accelerated cure speed and water and other resistance properties after cure.